The invention disclosed herein is an improvement over U.S. Pat. No. 4,509,426, dated Apr. 9, 1985 and over pending patent application Ser. No. 892,901, filed Aug. 4, 1986. The entire disclosures of the patent and application are incorporated herein by reference.
U.S. Pat. No. 4,509,426 describes a basic mechanism for causing an ink roller which is included in the inker of an offset printing press to rotate and oscillate axially as a result of being driven rotationally by another power driven roller that is in tangential contact with the axially oscillating roller. In addition to be usable in a printing press, the roller described herein can also be used in a machine that applies a coating to sheet material or a continuous web of material such as paper. Concentric to the shaft inside of the roller in the cited patent there is a sleeve in which there are axially adjacent left hand and right hand internal threads. A rocking arm is mounted to the shaft for pivoting about an axis that is transverse to the axis of the shaft. At each end of the rocking arm, there is a plunger, each of which has a projection constituting a thread follower element. The arm is automatically rocked so that when the follower on one plunger engages with the left hand thread the roller will move axially in one direction while the other follower is disengaged. Upon reaching the desired limit of axial movement, the engaged follower encounters a striker which expels the follower from its cooperating thread and causes the other follower to engage the other internal thread, thereby causing the roller to instantaneously reverse its direction of axial movement.
Although the roller has distinguished itself commercially by reason of getting more uniform ink distribution in the inkers of printing presses and although it has eliminated ghosting in cases where other approaches have failed, the roller oscillating mechanism is not optimized for use in the recently inaugurated highest speed presses. For one thing, frictional forces are not minimized. These frictional forces occur where the plungers having the thread follower prongs slide along guide cylinders. The rocker arm has a sphere at each end for registering in a socket in the plunger which it drives. The sphere and socket arrangement generates some friction but of even more concern is the requirement that the parts be carefully machined and aligned to properly mate with each other. But every machining step and every instance where careful fitting is required raises the cost and complicatedness of the design. The number of moving parts and the movement of one part upon another, of course, results in many opportunities for wear to occur which portends shorter operating life for the mechanism. Although the design was completely satisfactory for use in the previous generations of presses that fed sheet material through at about 600 feet a minute, it could be predicted that the design would not be suitable for the high speed presses which have been recently and are currently being installed. The highest speed presses presently available are often operated at the web feed rate of 2,000 to 2,200 feet per minute. The problems of designing a durable oscillating mechanism can be appreciated when one recognizes that in the highest speed presses the axially oscillating roller would be required to complete something over 12,000 cycles per hour or, in other words, over 24,000 reversals per hour. A roller may move axially in the range of three-quarters of an inch to two inches at high axial speed and then reverse instantly and accelerate to full speed in the opposite direction substantially instantly. Sometimes a production run on a press can go on for 24 hours a day for a full week or even more. The number of axial reversals that a roller experiences under such circumstances is almost impossible to comprehend.
Progress in upgrading the durability of the axial oscillating mechanism is illustrated in the above-cited application Ser. No. 892,901. In the design disclosed therein, short bell cranks are used to drive the followers and a sliding link is substituted for a rocking arm. When a striker in an internal left or right hand thread struck the follower, it would be forced down and would rock its associated bell crank. This, in turn, would cause the sliding link to shift and rotate the bell crank that is engaged with the other plunger follower, thus driving it into engagement with the internal thread of opposite twist so as to cause the roller to reverse its direction of axial movement. In this design, the follower plungers had at least two thread segments formed on them. These segments had the same pitch as the left and right hand internal threads to obtain a reduction in the force per unit area between the internal threads and the follower thread segments. This design has advantages and disadvantages. The disadvantages are that it increases the number of parts used in the oscillating mechanism rather than decreases the number of parts compared to the basic patented design. The intended simplification was not achieved nor was there any reduction in assembly time nor significant extension in operating life.